Solar energy is arguably our most viable low cost energy
source. It is forever sustainable and easily captured and converted. But now
the technology may have taken yet another leap forward. To date the foundational
technology behind photovoltaics was a structure called perovskite, which has
been made with lead. Using tin instead of lead perovskite as the harvester of light,
a team of Northwestern
University researchers has created a new solar cell with "good efficiency".
This good efficiency solar cell is low-cost, environmentally friendly and can
be easily made using "bench" chemistry -- no fancy equipment or
hazardous materials.

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"This is a breakthrough in taking the lead out of a very
promising type of solar cell, called a perovskite," said Mercouri G.
Kanatzidis, an inorganic chemist and tin expert. "Tin is a very viable
material, and we have shown the material does work as an efficient solar
cell."

Solar cells have typically used a structure called a perovskite made
with lead as the light-absorbing material. The new solar cell is made with tin
instead. Lead perovskite has achieved 15 percent efficiency. It is expected
that tin perovskite will reenergize the field by matching and possibly surpassing
that efficiency making them the "next big thing in photovoltaics".

Kanatzidis developed, synthesized and analyzed the material and then
turned to Northwestern collaborator and nanoscientist Robert P. H. Chang for
help in engineering a solar cell that worked well.

"Our tin-based perovskite layer acts as an efficient sunlight
absorber that is sandwiched between two electric charge transport layers for
conducting electricity to the outside world," said Chang. Details of the
lead-free solar cell will be published May 4 by the journal Nature Photonics.

Kanatzidis touts their solid-state tin solar cell efficiency rating of
just below 6 percent as a very good starting point. Two things make the
material special: it can absorb most of the visible light spectrum, and the
perovskite salt can be dissolved reforming upon solvent removal without
heating.

"Other scientists will see what we have done and improve on our
methods," Kanatzidis said. "There is no reason this new material
can't reach an efficiency better than 15 percent, which is what the lead
perovskite solar cell offers. Tin and lead are in the same group in the
periodic table, so we expect similar results."

Perovskite solar cells have only been around -- and only in the lab --
since 2008. In 2012, Kanatzidis and Chang reported the new tin perovskite solar
cell with promises of higher efficiency and lower fabrication costs while being
environmentally safe.